This invention relates to pre-amplifiers which are located at the focal plane of a photo-detector array. Each such pre-amplifier receives its input signal from a specific detector (or detectors) in the array, and transmits its output signal to the next electronic component, e.g., a bandpass filter.
In U.S. Pat. No. 4,555,623, assigned to the assignee of this application, the requirements of focal plane pre-amplifiers are discussed at length. Among the limiting factors identified in that patent are: (1) space limitations; (2) low power limitations; (3) high gain requirements; (4) DC bias requirements; (5) low noise requirements; and (6) variable gain adjustability.
The present focal plane pre-amplifier is intended to improve on that disclosed in U.S. Pat. No. 4,555,623 in at least three respects. (1) It reduces the space, or "real estate" required for each pre-amplifier. (2) It substantially eliminates the DC bias, or offset, of the pre-amplifier. (3) It substantially eliminates the 1/f, or flicker, noise in the pre-amplifier output.
One of the proposed techniques for substantially eliminating both the DC noise component due to offset, and the AC noise component due to temporal variations, is referred to as chopper stabilization. This technique reduces low frequency noise by shifting the frequency of the noise to a higher frequency outside the passband.
The present invention provides a simpler approach to the problem, which is particularly appropriate for incorporation in focal plane electronics.
The concept of chopper stabilization is discussed in an article titled "A Low-Noise Chopper-Stabilized Differential Switched-Capacitor Filtering Technique" (by Hsieh, Gray, Senderowicz and Messerschmitt) in the 1981 IEEE Journal of Solid State Circuit, Vol. SC-16, Number 6, December 1981, at pages 708-715. That article discusses various means of noise reduction, including these three: (1) large input geometries; (2) buried channel devices; and (3) chopper-stabilized frequency translation. It recommends the chopper stabilization technique for the particular usage of concern to the authors. The article also mentions a technique which it calls "correlated double sampling". However, it rejects that approach to the problem because it "requires that the output mode of the amplifier slew back and forth between the signal level and the initialized level each clock period. This puts a severe requirement on the operational amplifier settling time."